1. Field of the Invention
The present invention relates to an optical system for shaping light beams and an optical pickup employing the same, and more particularly, to an optical system for shaping a light beam output from a light source into a desired form and an optical pickup employing the same.
2. Description of the Related Art
An optical pickup for an optical storage medium, such as a compact disk (CD) and a digital versatile disk (DVD), uses a laser source for outputting a light beam having an elliptical cross-section. The laser source outputs light generated by an active layer of a laser diode in the form of a divergent beam. Referring to FIG. 1, generation of the laser light is briefly explained.
FIG. 1 shows an elliptical light beam output from a laser diode. In FIG. 1, a direction of a junction surface in the laser diode, that is, a direction parallel with an active layer is expressed as ".parallel." and a direction perpendicular to the junction surface is expressed as ".perp.". Here, the direction ".perp." coincides with the direction of current flowing through the active layer in the laser diode. In the case of a laser diode (Model No. PS010-00 manufactured by Blue Sky Research), an active layer region has a size of 1 .mu.m (direction .perp.).times.3 .mu.m (direction .parallel.) centered at a point B shown in FIG. 1. The laser light is generated from the active layer region. Since the light output through the active layer region starts at two different points A and B, the output light has astigmatic distance .DELTA.Z representing a distance between the points A and B. A divergence angle of the laser light is generally 20.about.40.degree. in case of .theta..sub..perp. and 8.about.20.degree. in case of .theta..sub..parallel., and thus, the output light beam has an elliptical cross-section with respect to the optical axis. Particularly, the long axis having a large beam diameter coincides with the direction .perp. and the short axis having a small beam diameter coincides with the direction .parallel. parallel with the junction surface.
However, since an objective lens for a light storage medium is circular, a light beam having a circular cross-section is needed to enhance a light utilization efficiency. The conventional beam shaping methods proposed by this need are described below with reference to FIGS. 2A through 4.
The optical system shown in FIGS. 2A and 2B includes two cylindrical lenses 11 and 12. FIG. 2A shows the lenses 11 and 12 viewed from the plane coincident with the direction .parallel. and FIG. 2B shows the lenses 11 and 12 viewed from the plane parallel with the direction .perp.. The lenses 11 and 12 have a different focal length, respectively. The diverging light beam output from the laser source as referred in FIG. 1, is collimated by a collimating lens (not shown) and then is incident to the lens 11. The lens 11 has a plano-concave shape at the direction coincident with the direction .parallel., to accordingly diverge the light incident thereto in parallel with the direction .parallel.. The lens 11 transmits the light incident in parallel with the direction .perp. without refraction. The light output from the lens 11 is incident to lens 12. The lens 12 outputs the light incident from the plano-concave lens 11 in parallel with the direction .parallel. in the form of substantially parallel light. The light incident at the direction parallel with the direction .perp. is transmitted without refraction via the lens 12, to accordingly keep it substantially parallel light. Thus, at the direction parallel with the direction .parallel. shown in FIG. 2A, the incident light having a beam diameter Wi is changed into that having a larger beam diameter Wo. As a result, the elliptical light beam output from the laser source is shaped into the light beam having a substantially circular cross-section.
FIG. 3 shows a conventional beam shaping prism 21. A light beam incident to the prism of FIG. 3 is a light beam elliptically output from a laser source and then collimated by a collimating lens, as in FIGS. 2A and 2B. The collimated light beam is incident to a surface 23 of the prism 21. At the incident plane shown in FIG. 3, the light beam of the small diameter direction having an incident angle .theta..sub.i is refracted by a refractive angle .theta..sub.o by the prism 21 having a refractive index n and then is output from the surface 25. The prism 21 changes the diameter Wi of the light beam incident to the incident plane of FIG. 3 into a larger diameter Wo. However, the prism 21 does not nearly change the beam diameter of the light incident to another incident plane perpendicular to the incident plane. Thus, the light beam output from the surface 25 becomes substantially circular.
FIG. 4 shows a conventional optical system for shaping a beam using a micro-lens 42. The light output from an active layer 41 has an elliptical cross-section as described above referring to FIG. 1. The light is incident to the micro-lens 42 distant by several micrometers from the active layer 41. The micro-lens 42 has an optical feature in which the light incident with respect to the small diameter direction as shown in FIG. 4 as dotted lines is transmitted without being substantially refracted. However, with respect to the large diameter direction shown as a solid line, the micro-lens 42 refracts the incident light via a convex surface 421 to become substantially parallel light, and diverges the light via a surface 423 to be substantially coincident with the beam diameter to the direction of the small beam diameter.
Since it is difficult to manufacture the above-described cylindrical lenses having excellent wavefront aberration and to adjust an optical axis, the method using the cylindrical lenses is rarely used.
In the case of the prism, since a desired beam shaping operation can be done only when substantially parallel light is incident, a separate collimating lens is needed to collimate the diverging light beam output from the laser source, which causes a long light path distance and makes it difficult to manufacture a compact optical pickup.
In the case of the method using the micro-lens, the micro-lens should be assembled in the output window of the laser diode, which makes it difficult to assemble the micro-lens with the laser diode without being a laser diode manufacturer and raises a manufacturing cost. It is also difficult to manufacture a micro-lens having an excellent performance.